1. Field of the Invention
The present invention relates to computerized simulation of hydrocarbon reservoirs in the earth, and in particular to simulation of historical performance and forecasting of production from such reservoirs.
2. Description of the Related Art
So far as is known, the development of compositional reservoir simulators in the industry has been restricted to models discretized with a relatively small number of cells (of the order of 100,000). Models of this type may have provided adequate numerical resolution for small to medium size fields, but become too coarse for giant oil and gas fields of the type encountered in the Middle East and some other areas of the world, e.g., Kazakhstan, Mexico, North Sea, Russia, China, Africa and the United States. As a result of this, sufficient cell resolution was only possible at the expense of dividing the reservoir model into sectors. This, however artificially imposed flow boundaries that could distort a true or accurate solution.
Another standard practice in the industry has been that of “upscaling” detailed geological models. “Upscaling” is a process that coarsened the fine-cell geological discretization into computational cells coarse enough to produce reservoir models of more manageable size (typically in the order of 100,000-cells). Such coarsening inevitably introduced an averaging or smoothing of the reservoir properties from a geological resolution grid of tens of meters into a much coarser grid of several hundred meters. This practice made it virtually impossible to obtain an accurate solution for giant reservoirs without excessive numerical dispersion. As a result, an undesirable effect was present—the geological resolution was being compromised at the expense of better fluid characterization.
Yet another compromise undertaken by the industry over the years has been that of performing a “semi-compositional” simulation, by which an equation of state program was used to provide compositional properties to a black-oil simulator while only solving the flow equations for three components (oil, water and gas). This approximation simplified the heavy computational burden at the expense of limiting itself to those problems where compositional changes in the reservoir were small and did not require full tracking of the flow of individual components, such as in U.S. Pat. No. 5,710,726.